Even if Faye develops from the silly girl she appears at first to one more alike to Spike, I don't think the character of Faye can compete with the mistery of Julia and her coldness. That fits him much better than Faye.
Faye can compete with the girl from the film, though, winning I think. The girl whose name I've forgotten would be too much alike, while Faye can give him a balance with her personality.
Her development however is a greatly done one, it served to make me dismiss her annoyance retroactively apart from making her go from the annoying sillyness of the first chapters she appears into something more serious and fleshed.